It is conventional practice in radiation therapy to cast beam-forming shielding blocks specifically individualized to each patient. The shielding blocks are usually cast in a polyfoam block such as polystyrene which has been especially cut to make the mold. One method of making the shielding block molds is to place the patient x-ray a set distance from a pivot which simulates the radiation source and to place a block of rigid polyfoam a set distance between the x-ray and the pivot and to cut the polyfoam with a cutter which follows either a pivoted rigid rod or long wire cable as the operator traces the designated treatment areas on the x-ray.
The voids thus created in the polyfoam are then filled with a molten eutectic alloy which cools to become one or more shielding blocks. The shielding blocks are then placed on a treatment tray in the beam of the radiation therapy machine, such as an accelerator or cobalt unit. The shielding blocks are manipulated into position on the treatment tray to shield the areas of the patient which are to be protected from radiation.
While this current practice of defining the radiation beam is certainly better than the older practice of manually stacking and arranging lead blocks, the whole process leaves much to be desired, and it leads to many cumulative inaccuracies because the process described above cannot produce shields with sufficient accuracy for use with modern high-powered, well-collimated treatment machines. Even slight errors in block fabrication can seriously undermine the physician's intent by allowing some areas to go untreated while others are seriously over-exposed.
In the prior-art hot-wire block cutters, exemplified by Wilson, U.S. Pat. No. 3,968,711, Priestly, U.S. Pat. No. 3,017,487, and Kelsey, U.S. Pat. No. 3,540,336, the wire is electrically heated to some pre-selected "optimum" in order to cut the polyfoam. However, such a cutter cannot cut accurately, because the wire temperatures cannot be correct for all cutting speeds and depths of cut. Not only does the thickness change because of the changing angle, but the speed constantly changes as the operator traverses the complex pattern. Accurate cutting of complex detailed shapes is not possible, because the cutter either drags behind the operator or takes the path of least resistance, unable to keep pace with the constantly varying load placed upon it. Accordingly, it is the purpose of this invention to provide an improved system of producing and verifying, extremely accurately formed shielding blocks.